According to a charged particle beam apparatus such as an electron microscope which performs high-magnification observation using electron beams, images oscillate by external small sound pressure or oscillation. The damage caused by oscillation becomes more conspicuous as the magnification increases. For the purpose of preventing damage to images caused by application of installation environment noise, a noise-proof cover is equipped as means for interrupting transmission of sound waves applied to the apparatus. Generally, the noise-proof cover has surfaces of hexahedron structure constituted by upper and lower, left and right, and upper and lower surfaces, considering the characteristics of sound waves which can go around and enter, and further the easiness of construction and low cost.
For improving the noise proof performance of the cover, noise absorption inside the cover is effective. It is therefore effective to provide organic porous material throughout the inner surface of the cover. However, the charged particle beam apparatus is generally used within a clean room, in which case such a problem may arise that the dusting characteristics produced by splashes from the organic material deteriorate the dustproof condition of the clean room. For overcoming this problem, PTL 1 discloses a technology which covers a noise absorbing material by dustproof fibers and attaches the noise absorbing material to an external cover.
Furthermore, in the field of acoustic engineering, it is generally known that there exists a resonance frequency dependent on the shape of a flask-type vessel and produced by air oscillation at an opening of the shape of the vessel. This is called Helmholtz resonance theory, and technologies based on this theory have been developed for absorbing noise. As a structure utilizing these technologies, PTL 2 discloses a noise absorbing structure constituted by a box-shaped component having a number of small bores.